1. Field of the Invention
The present invention relates to a chromatic dispersion distribution measurement apparatus for measuring chromatic dispersion distribution in an optical communication path, such as an optical fiber or the like, and a method for the same.
2. Description of Related Art
In optical communication systems, the chromatic dispersion is one of the causes that prevent high-speed of transmission signals or the wavelength division multiplexing (WDM) transmission or the like. The chromatic dispersion is a phenomenon that each speed of the transmission signals which propagate through an optical fiber varies with each wavelength of the transmission signals. In order to suppress the deterioration of the transmission signals, which is caused by the chromatic dispersion, to a minimum, it is required to control the chromatic dispersion value in an optical transmission line. Therefore, in the construction of the optical communication systems, a technique of measuring precisely a chromatic dispersion characteristic has been important.
In earlier technology, as what measures the chromatic dispersion characteristic, for example, there is a technique of measuring dispersion distribution in a longitudinal direction of an optical fiber under test, which is disclosed in Japanese Patent Publication No. Tokukai-Hei 10-83006 (corresponding to the U.S. Pat. No. 5,956,131 and European Patent Application No. 0819926A2). In the disclosure, two optical pulses with distinct wavelength are inputted into an optical fiber under test, and a four wave mixing (FWM) light is generated according to the interaction of the two optical pulses with distinct wavelength. Then, a specific wavelength component is extracted from a back-scattered light of the FWM light by an optical band pass filter (BPF) and measured by an optical time domain reflectometer (OTDR).
However, in such a technique described above, there was a case that the dispersion distribution in the optical fiber under test itself could not be measured precisely. Particularly, when a long optical fiber is constructed, or the like, a plurality of short optical fibers are connected, and the loss (splice loss) is generated in the connecting points. Then, the phases of the optical pulses are fluctuated remarkably according to the intensity change generated by the loss. Therefore, there was a problem that the substantial chromatic dispersion value in the optical fiber is affected by the fluctuation (splice loss).
The present invention was made in view of the above-described problem. An object of the present invention is to provide a chromatic dispersion distribution measurement apparatus which can measure the chromatic dispersion in an optical communication path itself, such as an optical fiber or the like, in high accuracy by canceling the error caused by the steep loss, that is, caused by decreasing the intensity of lights dramatically, such as the splice loss or the like, in the optical communication path under test. Another object is to provide a method for the same.
In order to solve the above-described problem, according to a first aspect of the present invention, a chromatic dispersion distribution measurement apparatus for calculating a chromatic dispersion distribution in an optical communication path under test, comprises: an intensity ratio calculation unit for calculating an intensity ratio of a first light and a second light that are reflected from an optional position of the optical communication path under test and propagated to an incident end of the optical communication path under test by taking the same propagation time; and a chromatic dispersion value calculation unit for calculating a chromatic dispersion value in the optical communication path under test in accordance with the intensity ratio calculated by the intensity ratio calculation unit.
In the specification, the term xe2x80x9coptical communication path under testxe2x80x9d is a general term of all of the objects to be tested for the optical characteristics, such as an optical fiber, an optical cable or the like.
The first light may be reflected from the optical communication path under test when a first optical pulse having a single wavelength is directed on the incident end of the optical communication path under test, and the second light may be reflected from the optical communication path under test when a second optical pulse having a plurality of wavelengths which are reciprocally distinct is directed on the incident end of the optical communication path under test.
The chromatic dispersion distribution measurement apparatus may further comprise: a first measurement unit for measuring an intensity of the first light for each propagation time of the first light; and a second measurement unit for measuring an intensity of the second light for each propagation time of the second light. Further, the chromatic dispersion distribution measurement apparatus may comprise a storing unit for storing one of the intensity of the first light, which is measured by the first measurement unit, and the intensity of the second light, which is measured by the second measurement. The intensity ratio calculation unit may calculate the intensity ratio of the first light and the second light with reference to the intensity stored in the storing unit.
The chromatic dispersion distribution measurement apparatus may further comprise: a first light source for generating the first optical pulse and the second optical pulse; and a second light source for generating the second optical pulse. Further, the chromatic dispersion distribution measurement apparatus may comprise an optical switch unit for actuating the first light source and the second light source.
Moreover, logarithms of both an intensity of the first light and an intensity of the second light may be taken, and a difference between the logarithms may be calculated by the intensity ratio calculation unit so as to obtain the intensity ratio of the first light and the second light.
In the present invention, the first light may be a back-scattered light of a light having a single wavelength, and the second light may be a back-scattered light of a four wave mixing light.
According to the present invention, the intensity ratio calculation unit calculates the intensity ratio of the first light and the second light that are reflected from an optional position of the optical communication path under test and propagated to the incident end of the optical communication path under test by taking the same propagation time. Further, the chromatic dispersion value calculation unit calculates the chromatic dispersion value in the optical communication path under test in accordance with the intensity ratio calculated by the intensity ratio calculation unit. Therefore, an error caused by the splice loss or the like in the optical communication path under test can be canceled. That is, since both the intensity of the first light and that of the second light are influenced by the splice loss or the like, in the optical communication path under test, the splice loss or the like can be canceled when the ratio of these intensities is calculated. Therefore, the chromatic dispersion in an optical communication path under test itself can be measured in high accuracy.
According to a second aspect of the present invention, a chromatic dispersion distribution measurement apparatus comprises: a first measurement unit for measuring an intensity of a first back-scattered light for each propagation time of the first back-scattered light at predetermined sampling timing intervals, the first back-scattered light being reflected from an optical communication path under test when a first optical pulse having a single wavelength is directed on the optical communication path under test; a second measurement unit for measuring an intensity of a second back-scattered light for each propagation time of the second back-scattered light at predetermined sampling timing intervals, the second back-scattered light being reflected from the optical communication path under test when a second optical pulse having a plurality of wavelengths which are reciprocally distinct is directed on the optical communication path under test; an intensity ratio calculation unit for calculating an intensity ratio of the first back-scattered light and the second back-scattered light that are reflected from an optional position of the optical communication path under test and propagated to an incident end of the optical communication path under test by taking the same propagation time, in accordance with the intensity of the first back-scattered light, which is measured by the first measurement unit, and the intensity of the second back-scattered light, which is measured by the second measurement unit; and a chromatic dispersion value calculation unit for calculating a chromatic dispersion value in the optical communication path under test in accordance with the intensity ratio calculated by the intensity ratio calculation unit.
The chromatic dispersion distribution measurement apparatus may further comprise: a first light source for generating the first optical pulse and the second optical pulse; and a second light source for generating the second optical pulse. Further, the chromatic dispersion distribution measurement apparatus may comprise an optical switch unit for actuating the first light source and the second light source.
The chromatic dispersion distribution measurement apparatus may further comprise a storing unit for storing one of the intensity of the first back-scattered light, which is measured by the first measurement unit, and the intensity of the second back-scattered light, which is measured by the second measurement unit. The intensity ratio calculation unit may calculate the intensity ratio of the first back-scattered light and the second back-scattered light with reference to the intensity stored in the storing unit.
In the present invention, logarithms of both the intensity of the first back-scattered light and the intensity of the second back-scattered light may be taken, and a difference between the logarithms may be calculated by the intensity ratio calculation unit so as to obtain the intensity ratio of the first back-scattered light and the second back-scattered light. Further, the second back-scattered light may be a four wave mixing light.
According to the present invention, the first measurement unit measures the intensity of the first back-scattered light that is reflected from an optional position of the optical communication path under test when the first optical pulse having a single wavelength is directed on the incident end of the optical communication path under test, for each propagation time of the first back-scattered light at predetermined sampling timing intervals. The second measurement unit measures the intensity of the second back-scattered light that is reflected from an optional position of the optical communication path under test when the second optical pulse having a plurality of wavelengths which are reciprocally distinct is directed on the incident end of the optical communication path under test, for each propagation time of the second back-scattered light at predetermined sampling timing intervals. In this case, when a splice point or the like exists in the optical communication path under test, both the intensity of the first back-scattered light and that of the second back-scattered light are changed dramatically in the splice point.
Thereby, the intensity ratio calculation unit calculates the intensity ratio of the first back-scattered light and the second back-scattered light that are reflected from an optional position of the optical communication path under test and propagated to the incident end of the optical communication path under test by taking the same propagation time, in accordance with the intensity of the first back-scattered light, which is measured by the first measurement unit, and the intensity of the second back-scattered light, which is measured by the second measurement unit. Further, since the chromatic dispersion value calculation unit calculates the chromatic dispersion value in the optical communication path under test in accordance with the intensity ratio calculated by the intensity ratio calculation unit, an error caused by the splice loss or the like in the optical communication path under test can be canceled. That is, since both the intensity of the first back-scattered light and that of the second back-scattered light are influenced by the splice loss or the like in the optical communication path under test, the splice loss or the like can be canceled when the ratio of these intensities is calculated. Therefore, the chromatic dispersion in an optical communication path under test itself can be measured in high accuracy.
According to a third aspect of the present invention, a chromatic dispersion distribution measurement method comprises: calculating an intensity ratio of a first light and a second light that are reflected from an optional position of an optical communication path under test and propagated to an incident end of the optical communication path under test by taking the same propagation time; and calculating a chromatic dispersion value in the optical communication path under test in accordance with the calculated intensity ratio.
In the chromatic dispersion distribution measurement method, the first light may be reflected from the optical communication path under test when a first optical pulse having a single wavelength is directed on the incident end of the optical communication path under test, and the second light may be reflected from the optical communication path under test when a second optical pulse having a plurality of wavelengths which are reciprocally distinct is directed on the incident end of the optical communication path under test.
The chromatic dispersion distribution measurement method may further comprise: measuring an intensity of the first light for each propagation time of the first light; and measuring an intensity of the second light for each propagation time of the second light. Further, the chromatic dispersion distribution measurement method may comprise storing one of the measured intensity of the first light, and the measured intensity of the second light. The intensity ratio of the first light and the second light may be calculated in the intensity ratio calculating with reference to the stored intensity.
In the chromatic dispersion distribution measurement method, logarithms of both the intensity of the first light and the intensity of the second light may be taken, and a difference of the logarithms may be calculated in the intensity ratio calculating so as to obtain the intensity ratio of the first light and the second light.
In the present invention, the first light may be a back-scattered light of a light having a single wavelength, and the second light may be a back-scattered light of a four wave mixing light.